The present invention relates generally to a coating method and apparatus for magnetic recording media such as a magnetic tape and a floppy disk, and more particularly to such a coating method and apparatus for succesively forming two layers in a non-dried state.
In accordance with the requirement for heightening the performance of the magnetic recording media, there is recently noted a multi-layer-structurized magnetic recording medium such as a video magnetic recording medium having two magnetic layers. As the upper layer there is provided a magnetic layer for the high-density recording which has an excellent characteristic of magneto-electric conversion at a higher-frequency region, and as the lower layer there is provided a magnetic layer which has an excellent characteristic of magneto-electric conversion at a lower-frequency region. Unlike the conventional single-layer structure, this arrangement can realize a magnetic recording medium having an excellent characteristic of magneto-electric conversion over a wide region. In order to simplify the manufacturing process of such a two-layer video magnetic recording medium, it is preferable to form the two layers by performing the coating and drying at the same time as disclosed in the Japanese Patent Provisional Publication No. 62-124631.
There is a problem with such a conventional coating method and apparatus, however, in that fine stripes enlongated in the longitudinal directions of a support (a stripe pattern of uneven coating on the coating surface) can appear on the coated surface in cases where the high-density recording magnetic coating solution is coated repeatedly so as to form plural layers. This problem has been clearified by the study made by the inventors of this invention. FIG. 1 shows a coating result to be obtained by performing the calendering process after coating as the upper layer a magnetic coating solution as shown in a table 1 and successively coating as the lower layer a magnetic coating solution as shown in a table 2 on a polyethylene terephthalate film with a thickness of 14 .mu.m in accordance with the conventional coating technique the magnetic coating solution for the upper layer being coated thereon so that the layer thickness is 0.5 .mu.m in the dried state and the magnetic coating solution for the lower layer being coated thereon so that the layer thickness is 3 .mu.m in the dried state. In FIG. 1, the coated surface is measured by means of a 3-dimension surface profile analizer. For making obvious the height portions of the coated surface for the 3-dimensional display, there are outputted only the portions higher than the average value in the height direction. According to the measurement result, the stripe pattern is seen on the coated surface at a pitch of about 50 .mu.m in the longitudinal directions of the support (the base film). Further, in FIG. 1, the average surface roughness (RMS) of the coated surface has been seen to be 15.8 mm. When the characteristic of magneto-electric conversion is measured by means of a MII format deck, the video band output (7 MHz) is -2 dB with respect to the Applicant reference tape and the S/N ratio is -1 dB. It is clear from the aforementioned result that the longitudinally elongated stripes can extremely deteriorate the characteristic of magneto-electric conversion.
TABLE 1 ______________________________________ Magnetic Alloy Powder ______________________________________ Lengthwise Size 0.18 .mu.m 100 wt % Average Size Ratio 1:12 .sigma.s 128 emu/g Hc 1530 Oe Conductive Carbon 2 wt % Polyeurethane Resin 10 wt % Chloroethylene-vinyl acetate 10 wt % copolymer Alumina (.alpha.-Al.sub.2 O.sub.3) 5 wt % Stearic Acid 2 wt % Butyle Stearic Acid 2 wt % Methyle Ethyle Ketone 200 wt % Toluene 180 wt % Cyclohexanone 40 wt % ______________________________________
According to the research of the inventors of this invention, this is due to the following causes. That is, the magnetic powder particles of the magnetic coating solution tend to be combined with each other by means of the magnetic suction force, and therefore they are not presented as the first-order particles but form 3-dimensional network structures. In response to a shearing force being applied, the 3-dimensional network structures can be considered to be broken so as to form flocculations having a given size ("Tosokogaku", Vol.21 No.10 P475-479, 1986). Since the magnetic coating solution including a magnetic powder such as an magnetic alloy powder having a strong magnetic force or a magnetic powder having a small lengthwise average size of acicular particle has an extremely strong cohesive force, the above-mentioned flocculations can be considered to be presented in the flowing magnetic coating solution in orders of several tens to hundred .mu.m. When coating this magnetic coating solution on a support in accordance with the conventional coating technique, as illustrated in FIG. 2 the above-mentioned flocculation is pushed out from a liquid pocket of a die 5 onto a second lip 2. This causes the generation of the the stripe pattern of uneven coating on the coating surface. In FIG. 2, numeral 1 represents a first lip, 3 designates a manifold, 4 depicts a slit, 6 denotes a pump, 7 indicates a magnetic coating solution and 8 is a support. In addition, the generation of the stripe pattern tends to more easily and clearly occur as the magnetic powder of the magnetic coating solution to be coated has a stronger magnetic force and a smaller lengthwise average particle size of acicular particle. Recently, in order to meet the requirements to more heighten the recording density of the video magnetic recording medium, for the upper layer there is used a magnetic coating solution including an extremely fine magnetic powder having a high magnetic force. Thus, the characteristic of magneto-electric conversion such as the video band output and S/N is considerably deteriorated due to the stripe pattern of uneven coating on the coating surface, thereby dealing a fatal blow to the quality of the article.